1. Field of the Invention
The invention is concerned with butt welding sheets of thermoplastic material.
2. Description of the Prior Art
U.S. Pat. No. 3,769,124 issued Oct. 30, 1973, and U.S. Pat. No. 3,834,971 issued Sept. 10, 1974, described a technique for butt welding the leading edge of one sheet of rolled thermoplastic material to the trailing edge of another sheet of such rolled material. According to these prior applications, the contents of which are hereby incorporated by reference, the leading portion of a sheet of rolled thermoplastic material is transversely severed by contacting it with a moving hot element, the trailing portion of a second sheet of rolled thermoplastic material is also transversely severed by contacting it with further movement of the same hot element; the transversely severed sheet edges, in a heated condition are then butted and thus welded together.
While this process, and the apparatus which is used to carry it out, performs its intended function very well, on occasion it has been found to be difficult to uniformly heat the moving hot element to the proper temperature for the severing and heating, thereby preventing the apparatus from performing properly. Clearly, if the hot element is not heated to a temperature sufficient to cleanly sever the trailing and leading portions, and to heat the sections adjacent to the line of severance to a temperature sufficient to allow them to be butt welded together, the apparatus cannot efficiently carry out the process of butt welding thermoplastic material. Furthermore, if the hot element temperature is not substantially uniform, the severing and heating functions of the element may only be partially performed, as a portion of the element has not reached a sufficiently high temperature to perform these functions. Such a temperature non-uniformly might be caused by the turbulent air flow often present in the environment within which this type of apparatus operates. The varying velocities, temperatures and volumes of such turbulent air flows allow for the non-uniform cooling of the heated element, thus possibly producing an element whose average temperature is of the proper level to perform adequately according to this invention, but whose actual temperature at a given location may not be high enough to perform properly. Furthermore, the element is kept hot for a dwell time following the thermoplastic material severing and heating in order to burn, melt or vaporize off any thermoplastic material buildup. This burn off cannot be performed properly and completely if portions of the hot element are not of sufficiently high temperature.
A primary object of this invention is to control the temperature of a hot moving element so that the element's temperature is sufficient to sever thermoplastic sheets of material by passing the hot moving element through the thermoplastic material. Another objective of this invention is to control the temperature of a hot moving element so that the element's temperature is sufficient to heat thermoplastic material to a temperature sufficient to allow for two sheets thereof to be butt welded together to form a continuous sheet by passing the hot moving element adjacent to the sheets to be so joined. A further objective of this invention is to furnish means to keep the element temperature substantially uniform. Another object of this invention is to prevent the build-up of residual thermoplastic material on the hot moving element. Other and additional objects of this invention will become apparent from a consideration of this entire specification including the claims and drawings.